1. Field of the Invention
This invention relates generally to hydraulic servo-amplifiers and, more particularly, to a hydraulic linear servo-amplifier having a control valve that is controlled externally by a control rod.
2. Description of the Prior Art
There are already known hydraulic linear servo-amplifiers that include a piston that is arranged within a first cylindrical bore of a housing, in which a piston rod extends on either side of the actual sealing portion of the piston with one end of such rod extending out of the housing and having an end suitable for connection to the mechanism to be actuated. A control slide valve or spool valve is arranged in a bore in the piston rod, and the control slide valve is actuated by an operating rod that extends outside of the housing. An input connection for pressurized hydraulic fluid, for example, is connected with a first cylinder space at one side of the piston and a return outlet for such hydraulic fluid is provided in a second cylinder space at the other side of the piston. By providing lands and annular chambers of reduced diameter on the exterior cylindrical surface of the control slide valve, the valve then controls the flow of fluid from the first cylinder space into the second cylinder space. One example of such a hydraulic servo-amplifier is found in European Patent Application No. 0,088,017 published Sept. 7, 1983.
Such servo-amplifiers are particularly suitable to provide a small linear stroke and relatively low starting outputs. The control slide valve and operating rod are generally guided within a coaxial bore in the piston rod and the operating rod extends through the housing to the exterior so that it may be actuated by an electric motor, for example, a stepping motor. The control slide valve is then moved in accordance with the operating rod and the piston follows the linear movements of the control slide valve. The slide valve makes the appropriate connection between the fluid inlet and outlet and the chambers on either side of the piston ring by having reduced diameter portions that are defined by so-called guiding edges. Generally in this kind of servo amplifier, the ratio of the maximum flow cross-section by way of the guide edges to the piston surface is limited. In addition, the inflow or outflow of the hydraulic fluid to or from the respective so-called guiding edges take place through relatively narrow channels and the control rate of such known device is thereby substantially limited.
Other examples of servo amplifiers of this kind are found in U.S. Pat. No. 3,892,164 and 3,961,561. Nevertheless, these systems all exhibit the same disadvantages described above relative to the servo amplifier of the above-identified European Patent application.